Preparation of siliceous aerogels



Jan. 12, 1965 A. B. SCHWARTZ 3,165,379

PREPARATION OF SILICEOUS AEROGELS Filed March 14, 1962 s 9 |0 u :2 l3

Sol pH 2 "I g 0.3 W I S O Silica Conceniration in Hydrosol, g./liter INVEN TOR.

A/ber/ B. SUM/0r Al/omey United States Patent "we 3,165,379 PREPARA'I'IUN F SILICEOUS AERGGELS Albert E. Schwartz, Philadelphia, Pa., assignor to ocony Mobil Oil (Zompany, Inc., a corporation of New York Filed Mar. 14, 1962, Ser. No. 179,742 Claims. ((11. 23-182) This invention relates to a method for preparing a siliceous gel, one continuous phase of which is a gas, i.e., a siliceous aerogel. More particularly, the present invention is directed to a process for producing aerogels having a solids content consisting essentially of silica by reaction of an alkali metal silicate, in the absence of added acid, with a water-soluble ammonium saltunder particularly specified conditions of formation and treatment of the gel product so obtained.

Aerogels have previously been described in certain patents to Kis-tler, i.e., US. 2,093,454 and 2,249,767. In accordancewith the disclosure of such patents a silica hydrogel, formed by the action of an acid on sodium silicate consists of a continuous solid phase and a continuous liquid phase. The solid phase consists of a structure of silica fibers with the liquid phase being held between these fibers. If such a gel is dried at normal pressure a very marked shrinkage takes place and the resultant product is a heavy hard glass-like mass. The shrinkage is caused by the formation of a gas-liquid interface within the gel pores during the evaporation of the liquid. Surface tension forces existing at this interface are sufiicient to cause the fibers to pull together until the structure is sufficiently compressed to withstand such forces. If the gel is heated under sufficient pressure to provide evaporation within the gel, then no gas phase reached. At this point the liquid phase is converted to the gas phase instantaneously thus avoiding a gas-liquid interface. Once the gas phase has been reached, the gas may be withdrawn without causing any collapse of the solid structure. The resultant product is a light, slightly opalescent solid which may contain as much as 95 percent by volume of air. Since colloidal silica undergoes a change in the presence of water at elevatedtempera-tures resulting in greatly increased particle size, the aqueous phase of the hydrogel is ordinarily replaced with an organic liquid such as ethyl alcohol or ethyl ether before the autoclaving operation. The organic liquid is then removed at a temperature above the critical. At such temperatures no gas-liquid interface is formed and the gel is therefore dried without shrinkageleaving a dried gel of the same volume as the initially formed hydrogel.

The above described process has the disadvantage of being rather expensive since it entails the use of large amounts of organic liquid, a substantial proportion of which generally cannot be recovered. In addition, it is to be noted that silica aerogels usually cannot be produced from silica gels in which the liquid phase consists of water since such gels ordinarily undergo dissolution before the critical temperature is reached.

It has also heretofore been proposed to prepare aerogels in the form of spheroidal particles by initially forming spheroidal particles ofhydrogel and replacing the forms until the critical temperature of the liquid is 3,ih5,37 Patented Jan. 12, 1965 aqueous phase of the hydrogel particles with an organic liquid and thereafter evaporating such organic liquid at a temperature not below its critical temperature. While it is highly advantageous in some instances to prepare aerogels in the form of spheroidal particles, the abovedescribed process has had the same disadvantages as that of the above initially described process, namely of being relatively expensive in necessitating the use of large amounts of organic liquid which generally are not subject to recovery.

In an attempt to overcome the above-noted disadvantages, it has previously been proposed to prepare siliceous aerogels by initially forming a silica hydrogel upon admixture of sodium silicate with a mineral acid at a pH of about 3.5 to 4.5 and permitting the resulting hydrosol to set to a hydrogel. The silica hydrogel so obtained, which is substantially free of metallic cations, is then heated in a pressure resistant vessel Without substantially subjecting the gel to a compressive liquid-solid interface to remove the liquid aqueous phase of the gel. While such method is generally less expensive than the above-described process utilizing large amounts of an organic liquid, such method has the disadvantage of requiring the use of acid resistant reacting vessels and of costly high pressure equipment.

It is a major object of the present invention to provide a method for preparing silica-containing aerogels and particularly silica aerogels which are free of the disadvantages present in the above-described previous procedures. It is a further object of this invention to aiford an inexpensive method for producing siliceous aerogels. A still further object of the invention is the provision of a commercially attractive method for preparing silica aerogels in the form of spheroidal particles.

The above and other objects which will be apparent to those skilled in the art are realized in accordance with the method of this invention. Broadly stated, the method for preparing siliceous aerogels comprises the reaction of an alkali metal silicate, in the absence of added acid, with a gelling agent of a Water-soluble ammonium salt to form a siliceous sol characterized by a pH in the range of about 9.5 to about 11.2 and a silica concentration of between about and about 160 grams SiO per liter of hydrosol. Preferably, theaforesaid pH is in the approximate range of 10 to 11 and the hydrosol silica concentration between about and about grams SiO- per liter. The resulting siliceous hydrosol is permitted to set to a hydrogel upon passage of a suitable interval of time. The resulting hydrogel is thereafterwashed free of soluble salts and dried under conditions of substantially atmospheric pressure. The resulting product is a very low density siliceous gel having the characteristics of the above described aerogels, being easily pulverizable and possessing a non-glassy appearance. The product so obtained is useful commercially in such applications as insulation, flatting agents for varnishes, lacquers and enamels, reinforcing agents for plastics and rubber, thickening agents for printing inks, nonskid ingredients in floor waxes, mold lubricants, anti-caking agents in powders and various other applications wherein siliceous aerogels have found use.

V vcc. and generally'less than.0.3 gram/cc; V

trary, if the gels'are prepared outsidethe above ranges,

applic'ation o'f'the product; I

aerogel product is to be usedin catalysis either as a cam V The alkali metal silicate reagent used in preparing the siliceous sol in accordance with this process will generally be sodium .silicate. 'However, it is contemplated that other suitable alkali-metal silicates may likewise be employed, such as, for example, potassium silicate. The gelling agent utilized in preparation of the siliceous solis a water-soluble ammonium salt such as, for example,

ammonium acetate, ammonium sulfate, ammonium chlo-.

ride, ammonium carbonate, ammonium fluoride, ammonium bromide, ammonium nitrate and ammonium phosphate. p

The quantity of the ammonium salt gelling agent required for sol formation varies with the particular conditions used. However, it has been found essential'in accordance with the process of this invention that the ratio of ammonium salt gelling agent to alkali metal silicate be controlled to produce a sol having a pH in the approximate range ,0f 9.5 to 11.2 and preferably in the range of about to about 11. The relative proportions andconcentrations of the reagents are further controlled such that the. resulting sol has a silica concentrationof between about and about 160 and preferably between about and about grams Si0 per liter of sol. It has been 7 found that the lowerconcentrations of silica unexpectedly 7 give rise to silica products of actually higher gel density than obtained with sols having a silica concentration within the aforementioned range. It has further been established, as will be evident from data set forth hereinafter, that the sol pH is an important factor and that sols having a pH of less than about 9.5 yield a gel prod uct having a density considerably higher than obtained by'controlling the sol pH withinthe approximate range 059.5 to 11.2. At a pH greater than-about "11.2, the gel time becomes long and'the density of the resulting gel is likewise considerably higher than that achieved by controlling the solpH Within'the herein specified range of 9.5 g

to 11.2. a

' If the above specifications as regards sol'pH and silica concentration arejfollowed, the resulting hydrogels are a white andopaque andresult in a low density gel product 5 characterized by a particle density of less than-0.4 gram/ On the conthe hydro'gelwill be. translucent or 'clear and williresult in ahighdensity product. -Particle density, as-utilized herein, is the density in grams per cubic centimenterof a single particle, excluding the void space between particles p but including internal pore volume Bulk density, de-' '1 pending on the compactness of the solid particles, will be considerably less than the particle density.-

The resulting siliceous hydrogel,"whether not it has undergone base-exchange treatment, is .washed free of water-soluble material; The resulting washed 7 gel is thereafter dried generally in air or steam under conditions of substantially atmospheric pressure. The dried ;gel .may be tempered, if desired, depending on the application of the product. Drying of the hydrogel is generally carried out at a temperature between about- F. and about 350" F. until the product isfsubstantially free of moisture.

that spheroidal'bead-like particles. of 'hydrogel are formed upon gelation. e V 7 Thus, in a preferred embodimentv of'jtheinvention' the siliceoussolformed ispassed in a finelydividedjstate into' a water-immiscibleliquidand retained therein until gelation .occurs. The siliceous sol prepared inaccordance 'with the present process, having the above specified-pH and silicaconcentratioh, will not set instantaneously to a gelatinous mass but on the other hand; will set to a hydrogcl upon passage of a suitable interval of'time. "This time differential may be controlled by jvariation in the solids contentof the 'hydrosol, by' variation in the ratio of gelling; agentto alkali. metal silicate, andby regula-j -tion of the temperature. of the .sol 'andthe water immiscible' liquid into which-the hydrosol is-introduced. Such time difierential permits passing the hydrosol 'into'the Water-immiscible liquid so that thesolmay" assume the desired spheroidal shape and set to'a hydrogel during passage through the liquid. When the hydrosol is formed, a

into spheroidal: particles employing the above ftechnique,

the gelationtime is. suitablyless than20 seconds,

The solutions of alkali metal silicateiand gelling agent used in formation of theprfesent' hydrosols are preferably mixed and introduced'as globules r-intothe iwater-irrlmisci- Y ble, liliuid'. The water-immiscibleiliquid.rnaylbe main Q trained at an elevated temperature in order to obtain gela tion-within the desired time. It will accordingly be under: stood that the time during which thehydrosol and the1resulting hydroge'l remain in the waterrimmiscible liquid and The siliceous hydrogel obtained initially contains zeolitic alkali metal due tothe use of the alkali metal silicate reagent employed. Thus, when sodium silicate'is the reagent employed,fthe initially formed siliceous gel will contain zeolitic sodium. Such zeolitic alkali metal may be removed-from the siliceous gel by base-exchange, i.e.,

replacementwith hydrogen-,jammonium or other metal ion. :Such base exchange is not essential'in' obtaining a lowdensity aerogel product inaccordance with the process of thisinyention. However, removal of-zeolitic' alkali metal :mayi'be desirable or necessary depending on the lyst or as asupport foran appropriate catalytic' agent, it is generally desirable that the product be free'ofzeolitic alkali metal. I Where it is desirable to modify the chem- ;ical composition of the aerogel' by the introduction of other,v metal ions such may 'beaccomplished duringre: placement of the zeolitic alkali metal with a suitable base exchange solutioncontaining such other-desired metal ion. Where itis not desirable or necessary to introduce anothermetal ion, the base-exchange solution may be an .1 ammonium salt or an acid.

Thus, where the siliceous the temperature of such liquid are correlated to'obtain the desired particles and that-these conditionsare inversely may be decreased. V i

related'so thatif the temperature is increased, the time Generally,.hydrogelsprepared' by'the process described f as herein are characterized by a gelation time of. not more than two hours; Although it is to be realized that hydro-V gels having a longer time of set when de'siredmay also Jae be produced by the present method. The method of this invention isiparticularly suitable'for the production of hydrogels characterized by a time :of set in the range of 0.5 [0520 seconds, which hydrogels are capable of being formed into the above-describedjspheroidal particlesupon introducing'the hydrosol in theiform of globules into a Water-immiscible-medium and maintaining the'hydrosol globules in such medium until they Set to globules of 'hydrogel. I

7 While the water-immiscible liquidin which gela-tion takes place may have a density higher than the siliceous hydrogel particles in which instance the hydrogel par ticles risev'upwardly through the liquid, such method is ordinarily less preferredthan in the case' where the liquid 1 has a lower density, allowing tthe 'hydrosol to be introduced at the top of a colnmn thereofand the spheroidal hydro- Q gelparticles formed therein to descend to the bottom of such column. A particularly suitable water-insoluble medium comprises organic liquids such as kerosene, lubricating oil, gas oils, etc. of such viscosity and density characteristics that the siliceous hydrosol introduced therein in the form of globules will settle-at the rate such that the hydrosol undergoes gelation to spheroidal particles of hydrogel during passage through the liquid;

After base-exchange of the initially formed siliceous hydrogel particles, if such has been employed to remove zeolitic alkali metal, the hydrogel particles are water washed free of soluble matter. torymethod for washing the hydrogel is by percolation either by upward or downward flow of water. After washing, the hydrogel particles are dried under conditions of substantially atmospheric pressure at a temperature generally from about 150 to about 350 F. and then if desired, tempered at a temperature of from about 350 to about 1400 F. for 1 to 24 hours or more. Ordinarily, it is preferred to dry the hydrogel particles in air or an A particularly satisfaccolumn.

a globular form and set in about 2 seconds to spheroidal particles of hydrogel during passage through the oil The resulting hydrogel particles were baseexchanged with an aqueous solution containing 10 percent by weight of ammonium sulfate. The hydrogel'particles were thereafter washed'with water to remove watersoluble impurities. The hydrogel particles were then dried in superheated steam at 250 F. and subsequently tempered in air for '5 hours at 400v F. The final silica aerogel product in the form of spheroids had a particle density of 0.23 g./cc.

Examples 29 were carried out following the generalprocedure of Example 1. The results of these examples, together with that of Example 1' are set forth below in Table I:

TABLE I SiO Mols Sol Properties Particle Cone, Rea-gent/ Density Example Reagent g./l. sol. M01 NazO of Gel Temp, Gel Product,

F. Time, pH gJcc.

, Min.

136 1.01 43 0. 033 ll. 0 0.23 109 1. 26 43 0. 023 10. 7 1 0. 19 86. 3 2. 21 38 0. 022 9. 6 0.32 92. 8 1. 88 0. 025 9. 8 0. 32 133 1. 42 45 0. 01 10. 0 0. 24 87. 5 2. 39 46 0.028 9. 6 a 0. 34 97. 3 2. 37 41 0. 027 9. 7 0. 28 100 3. 44 I 0.'01 9. 8 0.30

atmosphere of superheated steam at a'slow rate since such manner of operation has been found to result in less.

breakage of the gel particles. Theternperingoperation when employed is ordinarily carried out-in air although other inert atmospheres may likewise be used. The par.-

. the tempering operation is generally between about 800 and about 1400 F. If desired, the siliceous hydrogel par- 1 Real Density=2.1l g./ce.; Surface Area=262 sq. m./g.; Pore Volume=4.8 ce./g.

0 monium salts generally and specifically ammonium aceticles may be treated with catalytic components priorto the drying, or drying and tempering operations and the composites so obtained may then be subjected to the above-described drying and tempering.

The following examples will serve to illustrate the method of this invention without limiting the same:

EXAMPLE 1 A silica aerogel in bead form, was prepared from the following reactants:

7 Solution A v Water solution of sod-ium silicate (N Brand) having an SiO to Na O'weight ratio of 3.22 and a specific gravity at F. of 1.167. V

' Solution B tate; ammonium sulfate, ammonium carbonate,'and ammonium fluoride a'r e suitable gelling agents for usein the methodof this invention. In each instance, a low density siliceous aerogel product was obtained.

The series of examples below will serve to illustrate the marked effects of sol pH-and silica concentration on the particle density of the aerogel product obtained:

EXAMPLE 10 The following reactants were employed:

Solution A 1 Water solution of sodium silicate (N Brand) having an Slo to Na;() Weight ratio of 3.22 and a specific gravity at 60 F. of 1.240: g Y I Solution B Water solution of ammonium acetate containing 60 percent by weight of ammonium acetate.

Solution A flowing at a rate of 215 cc. per minute was mixed in' a nozzle with Solution B flowing at a rate of 384 cc. per minute. The hydrosol so formed had a silica concentration of 85.1 grams per liter of sol. The moles of ammonium acetate per mole ofNa O was 12.9. The hydrosol having a temperature of 42 F. and a pH of 9.0 was allowed toset. The resulting product was base-exchanged with an aqueous solution containing 10 changed product was thereafter washed with water to remove water-soluble impurities, dried in. superheated steam for -4 hours at 250300 F. and finally temperedv in air at 400 F. for 5 hours. The silica aerogel product had; a particle density of O.34- g./cc. f

Examples 11-19 were carried out following the general The base-ex-v procedure of Example 10. The results of these examples are set forth below in Table I1:

, a a r a in the art'within the scope of thefollowing claims with; out departing from the spiritthereof.

TABLE II 7 s10, Mols s01 Gel Particle Example. Reagent 'Conc Reagent 'Iemp., Time, $013111 Density I g./l. Sol Per M01 F. Sec. Aerogel,

. N940 glee.

10 Ammoaum Acetate 85.1 12.9 42 0.5 9.0 .34

do 85.1 0. 47 30 0v 0. 0 0. 2s 85.2 2.01 37' 0.5 10.2 a 0.21 85.1 1. 01 38 21 10.9 027 Efiect of SilieaConeentration in Hydrosol' Ammonium Acetate .125 1.01 35. 1.0 c 11.0 0.23?

It will be seen from the results of Examples 10-15 that I claim: 0 V a V the sol pH had a very marked effect on the density of the 25 1. A method for prepar ng a siliceous aerogel whlch geljproduct. Such efiect is particularly evident from a consideration of FIGURE'I of the drawing Where particle density of the gel product isplotted against the pH 4 Referring to such figure, it will be immediately 1 of the sol. apparent that a product of lowest density was obtained at a pH between 9.5 and 11.2 and particularly between 10 and 11. It will further be seen from the results of Examples 16-19 'thatthe silica concentration; of the'sol likewise had a very noticeable effect on the density, of the gel product. the drawing where particle density of the gel product is plotted against the-silica concentration ofthehydrosoli.

This'e'flectis evident from FIGURE 2 of Referring tothis figure, it will be seen that anunexpected minimum density for'the geljproduct Was obtained at a concentration between about and about 160 and. pref,-

erably between about and about 13 5 grams of silica per liter of solat the'particular sol pH of about. 11." At other solpH.si within the range of 915' to'11'. 2, the minimum density will be. achieved at other specified silica concentrations within the approximate range. of 80' to 160. 1 grams SiO per liter of sol. r

7 XA PLEsm-mj f v A series of aerogels was made in which the hydrogel was'base-exchanged with'various media. The gel wasprepared batch-wise and contained 108 grams SiO per liter of sol. acetate (1 mole ammonium acetate per mole"Na 0). The reactants were combined at 43 F. to form a sol having a pH of 11.0-and a gel time of 6 seconds; After The gelling'agent :employed'was ammonium base-exchange, thegels were washed, driediand tempered as in the preceding examples. ,The results obtained' are set forth in Table III'below:

It will be noted from the results set forth in the above table thatbas exchange with the various media had little or no efieetron the particl density 01 the final aerogel produet. p v v V This application is a continuation-in-part, ofcopending application Serial No. 781,328, filed December '18, 1958. I

It will be understood that the above description islmerely illustrative. of preferred embodiments of this invention of which .rn'a'ny variations may be made by those skilled I. comprises reacting an alkali metal silicate, in'the. absence 7 of added, acid, 'witha' gelling agentofa water-soluble'amammonium'salt being'such as to form asiliceous hydrosol 3 characterized by a pH'in the approximate'range of 9.5 to

about 160 grams SiO per. liter of hydrosol, permitting the resulting hydrosol toset'to'a'hydrogel, washing the hydrogel free of water-solubl ematter and drying there comprises reactingan alkali metal silicate, in the absenc e Y'of added acid, with a gellingagen't of awater-soluble ammoniunrsalt, the ratio of said alkali metal'silicateito said 1 ammonium salt'beingsuch as to form aisiliceous'jhydrosol V characterized .by' a. pH in the V approximate range "01:10 to L 'll-anda silica concentration of between {about 85 and about grams S10 per liter {of hydrosol', permitting the resulting hydrosol' to set. to 'a hydrogelgwashingthe hy- .monium salt, the ratio of said alkali metal silicatetoisaid' 11.2 and a silicaconcentrationof between aboutSOand.

' A methodfor preparing a siliceous aerogellwhich M drogelfree of water-soluble matter and drying the resultingprocluct, to thereby form a siliceousaero'gel Characterizedby a particle density;notexceeding about 0.3 gram per cubic centimeter. '3. Thefmethod of claim ammonium acetate. I V 7 method of claim' 1Wherein the ammonium salt 4. The isammonium sulfate. 1

is ammonium carbonate. f i I, 6. The method of claiml whereinthe lamrno'nit'im salt isammonium fluoride.

7. A method for preparingla 'siliceouslaerogel comprises reacting an alkalin etal'silicate; in the; absence moniumsalt, the ratio of said alkalimetal silicate'to said ammonium salt being such as to form a siliceousghydrosol 1 Whereirrjthe ammoniumsalt is V 5. The; method of claim 1 ma t c amniouiumsalt of added acid, withaigelling agentota water soluble am-' characterized by a pH in the approximate range of 9.5 to

the resultingihydrosol toysetto a hydrogel', washing the hydrogel free of water-soluble matter, drying the L'product pressure at' atemperature between about l 50'and about 7 0" 350F. and tempering theldried p'roductatYa temperature of-from about 350 to about 11400? F. frorn l to 24 "hours,

11.2 and asilica concentration of betweenabout 80. and about grams S10 peiizliter of, hydrosol, permitting so obtained under conditions of substantially atmospheric 8; TA process for forming spheroidal particles lofja sili-f a ceous aerogel which comprises reacting an alkali metal silicate, in the absence of added acid, with a gelling agent of a water-soluble ammonium salt, the ratio of said alkali metal silicate to said ammonium salt being such as to form a siliceous hydrosol which has a pH in the approximaterange of 9.5 to 11.2 and a silica concentration of between about 80 and about 160 grams SiO per liter of liydrosol, introducing said hydrosol as a finely divided stream into a water-immiscible medium wherein the hydrosol sets to globules of hydrogel, Washing the resulting hydrogel globules free of water-soluble material and drying the resulting product so obtained under conditions of substantially atmospheric pressure, to thereby form a siliceous aerogel characterized by a particle density not exceeding about 0.3 gram per cubic centimeter.

- 9. A method for preparing spheroidal particles of a siliceous aerogel which comprises reacting sodium silicate, in the absence of added acid, with a gelling agent of a water-soluble ammonium salt, the ratio of said alkali metal silicate to said ammonium salt being such as to form a siliceous hydrogel which has a pH in the approximate range of to 11 and a silica concentration of be- 12. The method of claim 8 wherein the ammonium'salt is ammonium carbonate.

13. The method of claim 8 wherein the ammonium salt is ammonium fluoride.

14. A process for forming spheroidal particles of a silica aerogel which comprises reacting an alkali metal J silicate, in the absence of added acid, with a gelling agent of a water-soluble ammonium salt, the ratio of said alkali I metal silicate to said ammonium salt being such as to form a siliceous hydrosol which has a time of gelation of less than about seconds, a pH in the approximate range of 9.5 to 11.2 and a silica concentration of between about and about 160 grams SiO per liter of hydrosol, introducing said hydrosol as a finely divided stream into a water'immiscible medium wherein the hydrosol sets to globules of hydrogel, washing the resulting hydrogel globules free of Water-soluble material, drying the product so obtained under conditions of substantially atmospheric pressure at a temperature between about 150 and about 350 F. and temperingthe dried product at a temperature of from about 350 to about 1400 F. for 1 to 24 hours, to thereby form a siliceous aerogel characterized by a particle density not exceeding about 0.3 gram per cubic centimeten 15. A process for forming spheroidal particles of a silica aerogel which comprises reacting an alkali metal silicate, in the absence of added acid, with a gelling agent of a water-soluble ammonium salt, the ratio of said alkali metal silicate to said ammonium salt being such as to form a siliceous hydrosol which has a time of gelation of less than about 20 seconds, a pH in the approximate range of 10 to 11 and a silica concentration of between about and about grams 510; per liter of hydrosol, introducing said hydrosol asa finely divided stream into a Water-immiscible medium wherein the hydrosol sets to globules of hydrogel, Washing the resulting hydrogel globules free of water-soluble material, drying the product so obtained under conditions of substantially atmospheric pressure at a temperature between about and about I 350" F. and tempering the dried product at a temperature of from about 350 to about 1400" F. for 1 to 24 hours, to thereby form a siliceous aerogel characterized by a particle density not exceeding about 0.3 gram per cubic cen timeter.

References Cited in the file of this patent UNITED STATES PATENTS 1,270,093 Arsem et a1 June 18, 1918 1,687,919 Yablick Oct. 16, 1928 2,114,123 Heuser Apr. 12, 1938 2,386,810 Marisic et a1, Oct. 16, 1945 

1. A METHOD FOR PREPARING A SILICEOUS AEROGEL WHICH COMPRISES REACTING AN ALKALI METAL SILICATE, IN THE ABSENCE OF ADDED ACID, WITH A GELLING AGENT OF A WATER-SOLUBLE AMMONIUM SALT, THE RATIO OF SAID ALKALI METAL SILICATE TO SAID AMMONIUM SALT BEING SUCH AS TO FORM A SILICEOUS HYDROSOL CHARACTERIZED BY A PH IN THE APPROXIMATE RANGE OF 9.5 TO 11.2 AND A SILICA CONCENTRATION OF BETWEEN ABOUT 80 AND ABOUT 160 GRAMS SIO2 PER LITER OF HYDROSOL, PERMITTING THE RESULTING HYDROSOL TO SET TO A HYDROGE, WASHING THE HYDROGEL FREE OF WATER-SOLUBLE MATTER AND DRYING THE RESULTING PRODUCT, TO THEREBY FORM A SILICEOUS AEROGEL CHARCTERIZED BY A PARTICLE DENSITY NOT EXCEEDING ABOUT 0.3 GRAM PER CUBIC CENTIMETER.
 8. A PROCESS FOR FORMING SPHEROIDAL PARTICLES OF A SILICEOUS AEROGEL WHICH COMPRISES REACTING AN ALKALI METAL SILICATE, IN THE ABSENCE OF ADDED ACID, WITH A GELLING AGENT OF A WATER-SOLUBLE AMMONIUM SALT, THE RATIO OF SAID ALKALI METAL SILICATE TO SAID AMMONIUM SALT BEING SUCH AS TO FORM A SILICEOUS HYDROSOL WHICH HAS A PH IN THE APPROXIMATE RANGE OF 9.5 TO 11.2 AND A SILICA CONCENTRATION OF BETWEEN ABOUT 80 AND ABOUT 160 GRAMS SIO2 PER LITER OF HYDROSOL, INTRODUCING SAID HYDROSOL AS A FINELY DIVIDED STREAM INTO A WATER-IMMISCIBLE MEDIUM WHEREIN THE HYDROSOL SETS TO GLOBULES OF HYDROG, WASHING THE RESULTING HYDROGEL GLOBULES FREE OF WATER-SOLUBLE MATERIAL AND DRYING THE RESULTING PRODUCT SO OBTAINED UNDER CONDITIONS OF SUBSTANTIALLY ATMOSPHERIC PRESSURE, TO THEREBY FORM A SILICEOUS AEROGEL CHARACTERIZED BY A PARTICLE DENSITY NOT EXCEEDING ABOUT 0.3 GRAM PER CUBIC CENTIMETER. 